(1) Field of the Invention
The present invention relates to an optical burst transmission system for intermittently transmitting data in optical signals, especially multiplexing and transmitting clock signals along with transmitting data.
(2) Description of the Related Art
FIG. 1 is a functional block diagram for an optical burst transmission system 1000 in the prior art. The optical burst transmission system 1000 comprises a multiplexing unit 1001, an address extracting unit 1002, an optical generator 1003, an optical modulator 1004, a first clock light generator 1005, a second clock light generator 1006, a third clock light generator 1007, an optical multiplexer 1008, an optical routing unit 1009, a first receiving unit 1020, a second receiving unit 1030, and a third receiving unit 1040.
FIG. 1 also shows, in addition to the aforementioned functional part, buffers 1101, 1102, 1103, 1201, 1202, and 1203 for storing data temporarily when data is transferred between the optical burst transmission system 1000 in the prior art and other input/output equipment.
The multiplexing unit 1001 reads from the buffers 1101, 1102, 1103 a plurality of data taking the form of packets, for example, three kinds of data to be transmitted to different destinations, Data A, Data B, and Data C, and multiplexes them into a transport stream.
The multiplexing unit 1001 also outputs a clock signal that is synchronous with the transfer rate of the multiplexed data to the first clock light generator 1005, the second clock light generator 1006, and the third clock light generator 1007.
The address extracting unit 1002 extracts address information to show the destinations of each data from the packet header of the multiplexed data.
The optical generator 1003 generates a kind of lightwaves each of which has a wavelength that is unique to each piece of the extracted address information so that the lightwaves function as data transmitting waves.
For each of the data packets, the optical modulator 1004 modulates the intensity of the lightwaves outputted from the optical generator 1003 according to the data included in each of the data packets, and outputs the lightwaves as “data light.”
The first clock light generator 1005, the second clock light generator 1006, and the third clock light generator 1007 generate another kind of lightwaves each of which has a wavelength that is unique to each of the destinations indicated by the address information and is different from that of the lightwaves generated by the optical generator 1003 so that this kind of lightwaves function as transmitting waves for the clock signals received from the multiplexing unit 1001 (hereafter this kind of lightwaves is referred to as “clock light”).
In other words, in the aforementioned case, three light emitting devices are needed for generating, in parallel, three lightwaves of clock light with independent wavelength for each of the destinations.
The optical multiplexer 1008 multiplexes the data light and the clock light and outputs the multiplexed combination to an optical transmission line such as a fiber optic cable.
The optical routing unit 1009 receives the combination of lightwaves from the optical transmission line and separates them into pairs of data light and clock light whose wavelengths are apart from each other with a predetermined interval called FSR (Free Spectral Range). The optical routing unit 1009 then guides the pairs of lightwaves to transmission lines each of which corresponds to the wavelengths of the pairs of lightwaves.
More specifically, for example, if the setting of the optical routing unit 1009 is such that only lightwaves with the wavelengths of λ10 and λ13 which are apart from each other with an FSR interval can be outputted from the first output port leading to the first receiving unit 1020, then the optical generator 1003 adjusts to λ13 the wavelength of Data A whose destination is the first receiving unit 1020, and the first clock light generator 1005 whose fixed wavelength is λ10 generates clock light to be used as the clock signal for Data A. Thus, the data light and the clock light for Data A are transmitted to the first receiving unit 1020.
Also, for other Data B and Data C, in the same manner, data light and clock light are generated with wavelengths that correspond to the destinations indicated by the address information included in these data, and transmitted to the destination receiving units.
The first receiving unit 1020 comprises a first optical demultiplexer 1021, a first clock detecting unit 1022, and a first data detecting unit 1023; the second receiving unit 1030 comprises a second optical demultiplexer 1031, a second clock detecting unit 1032, and a second data detecting unit 1033; and the third receiving unit 1040 comprises a third optical demultiplexer 1041, a third clock detecting unit 1042, and a third data detecting unit 1043.
Since the first receiving unit 1020, the second receiving unit 1030, and the third receiving unit 1040 have the same function, the following explanation uses the first receiving unit 1020 as an example.
The first optical demultiplexer 1021 receives the combination of lightwaves composed of the data light and the corresponding clock light of the data whose destination is the first optical demultiplexer 1021, and demultiplexes the combination into data light and clock light and outputs them.
The first clock detecting unit 1022 converts the demultiplexed clock light into a clock signal. The first data detecting unit 1023 converts the demultiplexed data light into data in the form of an electric signal. This data is stored in the buffer 1201 according to the converted clock signal. FIG. 2A shows how the data and the clock signal outputted from the optical multiplexer 1008 are transmitted.
The optical multiplexer 1008 outputs data light and clock light each of which has a wavelength that is unique and corresponding to the address information; for example, lightwaves with the wavelength λ10 (clock light) and λ13 (data light) for Data A whose destination is the first receiving unit 1020, lightwaves with the wavelengths λ11 and λ14 for Data B whose destination is the second receiving unit 1030, and lightwaves with the wavelengths λ12 and λ15 for Data C whose destination is the third receiving unit 1040.
FIG. 2B shows the optical filter transmission characteristics at the optical routing unit 1009. The optical routing unit 1009 has certain points where the transmission, intensity becomes the maximum for such wavelengths that are apart from each other with a predetermined interval, i.e. an FSR interval (hereafter, such wavelengths are referred to as “the peak transmission wavelengths”). The optical routing unit 1009 has a plurality of output ports for pairs of wavelengths whose intervals are the same FSR values and yet their absolute values of the peak transmission wavelengths are different from each other.
For instance, the first output port has its peak transmission wavelength at the wavelengths λ10 and λ13, and outputs only such lightwaves with the wavelengths at peak transmission wavelength.
In the same manner, the second output port outputs only such lightwaves with the wavelengths λ11 and λ14 which are at the two adjacent peak transmission wavelengths; and the third output port outputs only such lightwaves with the wavelengths λ12 and λ15 which are the two adjacent peak transmission wavelengths.
The destinations indicated by the address information and the output ports correspond one on one.
FIG. 2C shows the wavelength characteristics of the lightwaves outputted from the first output port. Because of the aforementioned features of the optical routing unit 1009, only lightwaves with the wavelengths λ10 and λ13 are outputted from the first output port leading to the first receiving unit 1020.
Here, the lightwave with the wavelength λ10 is clock light and the lightwave with the wavelength λ13 is data light.
Thus, the optical burst transmission system 1000 comprises, on an optical transmission line, the optical routing unit 1009 whose function is to use the wavelengths of the data transmitting lightwaves as an address for indicating the destination and to separate and output the optical signals from different output ports according to the wavelengths of the inputted lightwaves. This way, the optical burst transmission system 1000 performs routing, i.e. switching paths, in the optical area and makes it possible to transmit data at high speed and in a large quantity.
The optical burst transmission system 1000 of the prior art, however, has a problem that can be explained as follows: The optical routing unit has a feature to transmit only such pairs of lightwaves that have specific wavelengths different from each other. As shown in FIG. 1, when clock signals need to be transmitted to all destinations, there need to be a light source transmitting clock signals with a certain wavelength setting for every destination of the data, namely for every receiving terminals. Consequently, the more receiving terminals there are, the higher the costs of the transmission equipment, especially of the light sources will be.